Photoresist coated mask blanks are visually inspected with or without microscopes in the prior art filtered tungsten halogen lamps. For direct visual relection systems the lamp is placed at the focal point of a parabolic reflector. Blue light is absorbed by the filter to avoid exposure of the photoresist layer and infrared radiation is absorbed to minimize harmful heating effects, which expands the mask and causes misregistration thereby. The difficulty in using such a prior art reflectors are that (1) large reflectors and filters in the order of 6" (15 cm) in diameter are required to illuminate 100% of the surface of the mask; (2) only a very small percentage, namely in the order of about 10%, of the radiant energy emitted by the tungsten halogen lamp can be used for inspection purposes, resulting thereby in inadequate intensity of illumination and consequent low light scattering levels of visual defects; and (3) there is relatively short life in the order of 200 hours for the tungsten halogen lamp, resulting thereby in high maintenance costs.
For microscopic inspection systems, also using tungsten halogen lamps, defects as small as 1/2 micrometer can be detected. But such an inspection takes 30 minutes.
There is a need in the art for an improved technique for the visual inspection of masks of the type that are coated with a photoresist material.